This invention relates generally to nuclear reactors and more particularly, to flow measurement for reactor internal pumps in a boiling water reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The core center axis is substantially coaxial with the center axis of the shroud, and the shroud is open at both ends so that water can flow up through the lower end of the shroud and out through the upper end of the shroud. The shroud, top guide, and core plate limit lateral movement of the core fuel bundles.
The RPV also includes reactor internal pumps (RIP) located in the annulus between the shroud and the pressure vessel wall. The internal pumps provide circulation of water in the RPV. Typically the RIP flow is determined by measuring the fluid temperature, pump speed, and the pressure difference between four pairs of location points. One location point is upstream of the RIPs, and the other location point is downstream of the RIPs. The pressure difference verses flow correlation is based on simulated measurements in a test loop.
The upstream pressure measurement points are located in the annulus above the RIPs. The pressure and velocity gradients of the water in this region are small. That is, there is a negligible change in pressure with location in the horizontal X and Y directions, and the pressure change in the vertical direction corresponds to the change in the static head of water. This means that the measured pressure difference is not sensitive to exact location of the upstream measurement point. However, the situation for the downstream measurement point is very different.
The flow exiting the RIP diffuser is at high velocity and has a swirling pattern. The flow has to flow around the RIP impeller shaft and nozzle, and change directions from downward to radial inward to flow into the bottom head plenum. The flow pattern is very complex and turbulent. To overcome this turbulent flow problem, the downstream measurement point for known reactors is inside the shroud where the flow pattern is less complex and more steady. However, the pressure at this point is a function of the flow through several RIPs, not just one RIP, therefore, the performance of individual pumps in the reactor cannot be directly measured. In addition, the accuracy of the measurement is sensitive to the exact location of the downstream measurement. Further, the accuracy of the measurements depend upon how well the test loop replicates the reactor parameters and configuration. Changing the configuration of a boiling water reactor downstream of the RIP changes the calibration of the test loop.
It would be desirable to accurately measure flow through each RIP in a boiling water reactor based on a calibration curve that is independent of the geometry of the shroud support and bottom head plenum configuration.
A reactor pressure vessel for a nuclear reactor that permits measurement of the flow through each RIP, based on a calibration curve that is independent of the geometry of the pressure vessel, includes at least one pressure tap into each RIP. In an exemplary embodiment, the reactor pressure vessel includes a side wall, a reactor core shroud, a bottom head, and a bottom head petal attached to the bottom head. The bottom head petal includes a reactor shroud support flange, a reactor side wall support flange, and a reactor internal pump deck extending between the shroud support flange and the side wall flange. The pump deck having at least one opening extending therethrough to accommodate the RIPs.
The reactor pressure vessel also includes at least one reactor internal pump. Each RIP extends through a pump deck opening. Each pump includes an impeller and a diffuser. The diffuser includes a housing having an outer wall and a plurality of turning vanes that define a plurality of flow passages extending longitudinally through the housing. At least two seal rings extend circumferentially around an outer surface of the housing outer wall. The seal rings engage the inner surface of a pump deck opening to create a seal to reduce bypass leakage. Each seal ring is spaced apart from an adjacent seal ring in the longitudinal direction. The outer surface of the housing outer wall also includes a plurality of circumferential grooves. Each circumferential groove is sized to receive a seal ring.
At least one lateral bore extends through the housing outer wall into a diffuser housing flow passage. Each lateral bore is located in an area between two adjacent seal rings, with each inter-seal ring area containing one lateral bore. At least one pressure tap bore extends from the outer surface of the bottom head petal, through the pump deck to an inner surface of a pump deck opening. Each pressure tap bore is aligned with an area containing a corresponding lateral bore. The reactor pressure vessel also includes at least one pressure tap bore extending through the side wall of the pressure vessel. Each side wall pressure tap is located above the intake of a RIP.
To measure the flow through each RIP, a pressure measurement is taken at the pressure vessel side wall pressure tap upstream of the RIP. Also a pressure measurement is taken in the pump diffuser utilizing the pressure tap extending through the bottom head petal. The pressure differential is then calculated. The calibration curve created during the factory acceptance test of the RIP is used to translate pressure differential to pump flow rate.
The above described reactor pressure vessel and flow measurement system provides for calibrated flow measurements of each RIP. Also, the flow measurement system permits developing the flow verses pressure head curve during the factory test of each RIP without the need for a special test facility that replicates the reactor shroud support and bottom head geometry. Further, the seal rings, which extend circumferentially around the diffuser housing and engage the inner surface of a pump deck opening, permit disassembly of the pump for maintenance.